The same logic applies to the different castes. Eugenics or what means that 99% of Earth Caste are comfortable doing Earth Caste things. IQ, aptitude for intellectual activities, these characteristics have a genetic component. Sure, it's likely not all of them get the job that they want (interested in physics but gets stuck in chemistry) but that is how it works in real life too.
Saying that their society is oppressive because not everybody gets their dream job and dream life is very strange.